Even with modern equipment, digging tunnels is hard work. It took Swiss engineers 14 years to dig the world’s longest rail tunnel, the 57-kilometre Gotthard Base Tunnel under the Alps. It was finished in 2010, but construction began in 1996.

The Zambian mole rat doesn’t dig all the way through mountain ranges, but it does dig some of the longest tunnels in the natural world, and the longest of any non-human mammal. A single underground colony can stretch for 2.8 kilometres. Yet on average, each colony contains just nine or 10 mole rats. These guys are digging machines.

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Ratty mole

The Zambian mole rat is related to the famous naked mole rat, one of only two mammals to live in “eusocial” colonies&colon; like honeybees and termites, a single queen does all the breeding and the others work to maintain the colony. The other eusocial mammal is the Damaraland mole rat.

Zambian mole rats are less willing to sacrifice their own breeding rights for the good of the colony – colonies can have several breeding females – but they are still highly cooperative. Jan Šklíba of the University of South Bohemia in České Budějovice, the Czech Republic, and colleagues have carried out the first detailed study of wild colonies to find out how their societies work.

They mapped out 16 colonies. On average, each had 1.2 kilometres of tunnels and covered an area of 6919 square metres. For comparison, the largest known badger sett spanned just 879 metres. The Zambian mole rats’ tunnels were densely branched, particularly around the central nest where the breeding pair lived.

Yet each colony had on average 9.7 mole rats, each of which must have done an awful lot of digging. Naked mole rats produce tunnels of similar lengths, but their colonies average 80 individuals, which gives them an obvious numerical advantage.

Socialising?

Mole rats vary widely in how social they are, from the eusocial naked mole rats to the solitary small dune mole rat. According to Paul Sherman of Cornell University in Ithaca, New York, species living in harsh environments are more social. If the ground is hard to dig and food sources are widely separated, mole rats can only survive in groups.

The Zambian mole rats fit this to a tee, Sherman says. Šklíba found that the soil in the Zambian mole rats’ habitat was easier to dig than in the naked mole rats’ habitat. What’s more, although the tubers eaten by the mole rats were widely spaced, there were lots of them.

So the Zambian mole rats live in a moderately tough environment, but not as tough as the naked mole rats. In line with that, they’re social but not eusocial.

Šklíba also found evidence that the Zambian mole rats might be unusually friendly towards their neighbours. Nine of the colonies had a tunnel connecting them to one or two neighbouring colonies. In one case, four colonies were linked together in a daisy chain spanning 7.2 kilometres of tunnels.

Mole rat colonies are generally isolated from one another, but the Zambian mole rats may be an exception. Unpublished telemetry data suggests that members of one colony visited neighbouring colonies. Are they finding mates, socialising, or stealing food from rivals? Sherman says it’s also possible that the connecting tunnels are the mole rats’ way of starting new colonies. We don’t know yet, but it seems there’s an awful lot going on underground in Zambia.